Patent Application
20060131209
The present invention relates to an integrated refining process for the production of high quality gasoline blending components from low value components.
Modern refineries employ many upgrading units such as fluidic catalytic cracking (FCC), hydrocracking (HCR), alkylation, and paraffin isomerization. As a result, these refineries produce a significant amount of isopentane. Historically, isopentane was a desirable blending component for gasoline having a high octane (92 RON), although it exhibited high volatility (20.4 Reid vapor pressure (RVP)). As environmental laws began to place more stringent restrictions on gasoline volatility, the use of isopentane in gasoline was limited because of its high volatility. As a consequence, the problem of finding uses for by-product isopentane became serious, especially during the hot summer season. Moreover, as more gasoline compositions contain ethanol instead of MTBE as their oxygenate component, more isopentane must be kept out of the gasoline pool in order to meet the gasoline volatility specification. So, the gasoline volatility issue becomes even more serious, further limiting the usefulness of isopentane as a gasoline blending component.
The process of the present invention solves this problem by converting undesirable isopentane to low-RVP gasoline blending components by alkylation of the isopentane with a refinery stream containing ethylene using an ionic liquid catalyst. Other olefins, such as propylene, butylenes, and pentenes can also be used to convert isopentane to make low RVP hydrocarbon product. By reducing the excess isopentane, the burden of storing isopentane and/or concerns for isopentane usage are eliminated.
In general, conversion of light paraffins and light olefins to more valuable cuts is very lucrative to the refining industries. This has been accomplished by alkylation of paraffins with olefins, and by polymerization of olefins. One of the most widely used processes in this field is the alkylation of isobutane with C3-C5 olefins to make gasoline cuts with high octane number using sulfuric and hydrofluoric acids. This process has been used by refining industries since the 1940's. The process was driven by the increasing demand for high quality and clean burning high octane gasoline.
Commercial paraffin alkylation processes in modern refineries use either sulfuric acid or hydrofluoric acid as catalyst. Both of these processes require extremely large amounts of acid to fill the reactor initially. The sulfuric acid plant also requires a huge amount of daily withdrawal of spent acid for off-site regeneration. Then the spent sulfuric acid is incinerated to recover SO2/SO3 and fresh acid is prepared. The necessity of having to handle a large volume of used acid is considered a disadvantage of the sulfuric acid based processes. On the other hand, an HF alkylation plant has on-site regeneration capability and daily make-up of HF is orders of magnitude less. However, the aerosol formation tendency of HF presents a potentially significant environmental risk and makes the HF alkylation process less safe than the H2SO4 alkylation process. Modern HF processes often require additional safety measures such as water spray and catalyst additive for aerosol reduction to minimize the potential hazards.
Although these catalysts have been successfully used to economically produce the best quality alkylates, the need for safer and environmentally-more friendly catalyst systems has become an issue to the industries involved. The ionic liquid catalyst of the present invention fulfills that need.
In addition, implementing the present invention relieves a refinery of the problem and waste associated with excess fuel gas production. It does this by using ethylene in, for example, offgas from a FCC unit as the source of olefins for the alkylation of isopentane. Typically FCC offgas contains ethylene up to 20 vol %. Other olefin streams containing ethylene or other olefins such as coker gas could also be used for this process. The overall gasoline volume is increased by this process of invention. The net amount of fuel gas from the FCC de-ethanizer is reduced, thus lowering the burden of fuel gas processing equipment. A further benefit of the present invention is that extracting ethylene will improve the purity of hydrogen in FCC offgas. The improved concentration of hydrogen in the offgas may allow the economical recovery of pure hydrogen with the use of a hydrogen recovery unit, such as a pressure-swing adsorption (PSA) unit or a selective hydrogen-permeable membrane unit. Considering tight environmental regulations associated with fuel gas production and shortage of hydrogen in modern refineries, the benefits of fuel gas reduction and hydrogen production are highly desirable.
The most economical, thus most desirable, olefin streams are FCC de-ethanizer overhead containing hydrogen, methane, ethane, and ethylene, or coker gas containing olefins. The present process converts the isopentane stream to a low RVP dimethyl pentane and trimethyl butane gasoline fraction with little octane loss. By employing the process of the invention, the overall gasoline volume produced at a refinery is increased. In addition, the net amount of fuel gas from the FCC de-ethanizer is reduced, thus lowering the burden on the fuel gas processing equipment.
Furthermore, the present invention includes a new paraffin alkylation process which can produce alkylate gasoline, the most desirable blending component in gasoline, in an environmentally sound manner far superior to the conventional alkylation process. In comparison with the conventional processes, the process according to the present invention offers the following significant advantages over conventional alkylation:
It follows that employing a process according to the present invention a refiner can upgrade both isopentane and ethylene and at the same time react conventional alkylation feed components (e.g., butene, propylene, pentene, and isobutane) to produce high quality gasoline blending components. These additional capabilities are made possible in part with the high activity and selectivity of the ionic liquid catalyst used for these reactions. The present invention provides its greatest benefits when all these alkylation reactions are conducted with ionic liquid catalysts and none are conducted using sulfuric acid or hydrofluoric acid catalysts.
The present invention provides an integrated refinery process for the production of high quality gasoline blending components having low volatility comprising:
In addition the present invention provides a method of improving the operating efficiency of a refinery by reducing fuel gas production and simultaneously producing high quality gasoline blending components of low volatility.
The present invention also provides a high quality gasoline blending composition having low volatility prepared by the process described.
Feedstocks
One of the feedstocks to the process of the present invention is a refinery stream which contains olefins. Examples of such streams include FCC offgas, coker gas, olefin metathesis unit offgas, polyolefin gasoline unit offgas, methanol to olefin unit offgas and methyl-t-butyl ether unit offgas. The preferred olefin is ethylene. The preferred source of ethylene for conducting a process according to the present invention is offgas from an FCC unit, which may contain up to about 20 vol % of ethylene. This stream may also contain propylene, butylenes and pentenes. The FCC offgas is preferably passed through an ethylene extraction unit to produce a C2+ fraction, which is rich in ethylene, typically about 50 vol %, and a lighter fraction, which is rich in hydrogen. The C2+ fraction is fed to the alkylation reactor.
Another feedstock to the process of the present invention is a refinery stream which contains isoparaffins, preferably isopentane. Refinery streams which contain isopentane and which may be used in the process of the present invention include, but are not limited to extracted isopentane from an FCC unit, a hydrocracking unit, C5 and C6 streams from crude unit distillation, and extracted C5 and C6 streams from a reformer. Analysis of an extracted pentane sample from one refinery showed the feed stock to contain 86.4% iso-pentane, 8% n-pentane, 0.9% n-butane, 3.4% C6s-C9s and 0.2% olefins (C4 and C5 olefins). It also contained 88 ppm sulfur (mercaptans) and 0.4 ppm nitrogen. The feed stream exhibited very high RVP of 20, while the desirable current RVP target for gasoline is 7 to 8 range.
The isopentane-containing stream may also contain other isoparaffins such as isobutane. Isobutane may be obtained, for example, from hydrocracking units or may be purchased.
Catalyst
The use of ionic liquids as a new media and solvents for chemical reactions and particularly catalytic processes has gained wide popularity in the past several years. There has been an overwhelming surge in this research arena where ionic liquids have been used as solvents in an array of reactions such as olefins dimerization, olefin oligomerization and polymerization, isomerizations, alkylations, hydrogenations, Diels-Alder cyclizations and many others. In short, ionic liquids have been used as solvents in a wide range of organic reactions and processes.
A large number of liquid or solid acid catalysts are known which are capable of effecting alkylation of isoparaffins such as isobutane or isopentane by olefins such as propylene, 1-butene, 2-butene and isobutylene. The catalysts which are most widely used in industrial practice are concentrated sulfuric acid and hydrofluoric acid alone or mixed with Lewis acids such as boron trifluoride.
Those processes suffer from major disadvantages: hydrofluoric acid by virtue of its toxicity and its high degree of volatility; and sulfuric acid by virtue of a substantial volumetric consumption of the catalyst requiring burdensome regeneration. These reasons have motivated the development of catalysts which are solid or which are supported on solids such as aluminosilicates or metal oxides such as zirconia treated with sulfuric acid. However, solid catalysts are generally found to present a low level of selectivity and a low degree of activity. The use of aluminum chloride has also been studied and proposed.
The process according to the present invention preferably employs a catalytic composition comprising at least one aluminum halide and at least one quaternary ammonium halide and/or at least one amine halohydrate. The aluminum halide which can be used in accordance with the invention is most preferably aluminum chloride.
The quaternary ammonium halides which can be used in accordance with the invention are those described in U.S. Pat. No. 5,750,455, which is incorporated by reference herein, which also teaches a method for the preparation of the catalyst.
The ionic liquid catalysts which are most preferred for the process of the present invention are N-butylpyridinium chloroaluminate (C5H5NC4H9Al2Cl7). A metal halide may be employed as a co-catalyst to modify the catalyst activity and selectivity. Commonly used halides for such purposes include NaCl, LiCl, KCl, BeCl2, CaCl2, BaCl2, SiCl2, MgCl2, PbCl2, CuCl, ZrCl4, AgCl, and PbCl2 as published by Roebuck and Evering (Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, 77, 1970). Preferred metal halides are CuCl, AgCl, PbCl2, LiCl, and ZrCl4.
HCl or any Broensted acid may be employed as an effective co-catalyst. The use of such co-catalysts and ionic liquid catalysts that are useful in practicing the present invention is disclosed in U.S. Published Patent Application Nos. 2003/0060359 and 2004/0077914. Other co-catalysts that may be used to enhance the catalytic activity of ionic liquid catalyst system include IVB metal compounds preferably metal halides such as TiCl3, TiCl4, TiBR3, TiBR4, ZrCl4, ZrBr4, HfC4, HfBr4, as described by Hirschauer et al. in U.S. Pat. No. 6,028,024.
It is especially important to note that H2SO4 and HF are not effective for the alkylation of isoparaffins with ethylene. So, the process of the present invention would not have been considered in the past.
Reaction Conditions
Due to the low solubility of hydrocarbons in ionic liquids, olefins-isoparaffins alkylation, like most reactions in ionic liquids is generally biphasic and takes place at the interface in the liquid state. The catalytic alkylation reaction is generally carried out in a liquid hydrocarbon phase, in a batch system, a semi-batch system or a continuous system using one reaction stage as is usual for aliphatic alkylation. The isoparaffin and olefin can be introduced separately or as a mixture. The molar ratio between the isoparaffin and the olefin is in the range 1 to 100, for example, advantageously in the range 2 to 50, preferably in the range 2 to 20. In a semi-batch system the isoparaffin is introduced first then the olefin, or a mixture of isoparaffin and olefin. Catalyst volume in the reactor is in the range of 2 vol % to 70 vol %, preferably in the range of 5 vol % to 50 vol %. Vigorous stirring is desirable to ensure good contact between the reactants and the catalyst. The reaction temperature can be in the range −40° C. to +150° C., preferably in the range −20° C. to +100° C. The pressure can be in the range from atmospheric pressure to 8000 kPa, preferably sufficient to keep the reactants in the liquid phase. Residence time of reactants in the vessel is in the range a few seconds to hours, preferably 0.5 min to 60 min. The heat generated by the reaction can be eliminated using any of the means known to the skilled person. At the reactor outlet, the hydrocarbon phase is separated from the ionic phase by decanting, then the hydrocarbons are separated by distillation and the starting isoparaffin which has not been converted is recycled to the reactor.
Typical reaction conditions may include a catalyst volume in the reactor of 5 vol % to 50 vol %, a temperature of −10° C. to 100° C, a pressure of 300 kPa to 2500 kPa, an isoparaffin to olefin molar ratio of 2 to 8 and a residence time of 1 min to 1 hour.
A catalyst system comprised of aluminum chloride and hydrogen chloride (hydrochloric acid) for catalyzing the alkylation of iso-paraffins and olefins in ionic liquids (chloroaluminate ionic liquids) is preferred. The HCl can be used as a co-catalyst to enhance the reaction rate. For example, the alkylation of isopentane with ethylene in a batch autoclave is complete in <10 minutes in the presence of HCl. In the absence of HCl, the reaction usually takes ½ hour to 1 hour time (50° C. and autogenic pressure of ˜965 kPa and feed ratio of ˜4). The product selectivity was comparable to that of chloroaluminate ionic liquid without the presence of HCl.
Process Configuration
A scheme for an integrated refinery alkylation process to implement an embodiment of the present invention is shown in
An ethylene-containing refinery stream is fed to an Ethylene Extraction Unit to separate a C2+fraction rich in ethylene. The Ethylene Extraction Unit is typically comprised of membrane and/or distillation column separation equipment. A second refinery stream containing isopentane is fed to a Distillation Zone. Streams enriched in ethylene and isopentane are contacted in the presence of an ionic liquid catalyst in a Reactor under alkylation conditions. Then the catalyst and hydrocarbon phases are separated in a Catalyst Separator and the catalyst is recycled back to the Reactor. A portion of the recycling catalyst is sent to a Slip Stream Catalyst Regeneration unit. The hydrocarbon phase is sent to a Distillation Zone to recover unreacted isopentane for recycle, and the alkylate product is collected at the bottom. As needed, the alkylate product can be treated to remove any trace impurities.
The reject stream from the Ethylene Extraction Unit now has higher hydrogen purity. Further upgrading of the reject stream can be achieved by recovering pure hydrogen gas with use of a H2 Recovery Unit if desirable. The H2 Recovery Unit is typically comprised of a selective hydrogen-permeable membrane unit and/or pressure-swing adsorption (PSA) unit. A process according to the present invention offers a refiner considerable flexibility with respect to being able to prepare gasoline blending components of varying composition by selecting both the source of olefins used for alkylation and the paraffin-containing feedstock. Alkylation reactions in accordance with the present invention may be conducted in one or more alkylation zone using the same or different ionic liquid catalysts. For example, the C2+fraction described above may contain propylene, butylene and/or pentenes and the isopentane containing stream may also contain isobutane. Isobutane may be alkylated with ethylene to produce a high-octane C6 gasoline blending component. A C4 olefin containing stream may be isolated and used for the alkylation of isobutane, isopentane or their mixtures. Other variations and combinations will be apparent to refiners generally.
The following examples are illustrative of the present invention, but are not intended to limit the invention in any way beyond what is contained in the claims which follow.
N-butyl-pyridinium chloroaluminate is a room temperature ionic liquid prepared by mixing neat N-butyl-pyridinium chloride (a solid) with neat solid aluminum trichloride in an inert atmosphere. The syntheses of butylpyridinium chloride and the corresponding N-butyl-pyridinium chloroaluminate are described below. In a 2-L Teflon-lined autoclave, 400 gm (5.05 mol.) anhydrous pyridine (99.9% pure purchased from Aldrich) were mixed with 650 gm (7 mol.) 1-chlorobutane (99.5% pure purchased from Aldrich). The neat mixture was sealed and let to stir at 145° C. under autogenic pressure over night. Then, the autoclave was cooled down to room temperature, vented and the resultant mixture was transferred to a three liter round bottom flask. Chloroform was used to rinse the liner and dissolve the stubborn crusty product that adhered to the sides of the liner. Once all transferred, the mixture was concentrated at reduced pressure on a rotary evaporator (in a hot water bath) to remove excess chloride, un-reacted pyridine and the chloroform rinse. The obtained tan solid product was further purified by dissolving in hot acetone and precipitating the pure product through cooling and addition of diethyl ether. Filtering and drying under vacuum and heat on a rotary evaporator gave 750 gm (88% yields) of the desired product as an off-white shinny solid. 1H-NMR and 13C-NMR were ideal for the desired N-butyl-pyridinium chloride and no presence of impurities was observed by NMR analysis. N-butylpyridinium chloroaluminate was prepared by slowly mixing dried N-butylpyridinium chloride and anhydrous aluminum chloride (AlCl3) according to the following procedure. The N-butylpyridinium chloride (prepared as described above) was dried under vacuum at 80° C. for 48 hours to get rid of residual water (N-butylpyridinium chloride is hydroscopic and readily absorbs water from exposure to air). Five hundred grams (2.91 mol.) of the dried N-butylpyridinium chloride were transferred to a 2-Liter beaker in a nitrogen atmosphere in a glove box. Then, 777.4 gm (5.83 mol.) of anhydrous powdered AlCl3 (99.99% from Aldrich) were added in small portions (while stirring) to control the temperature of the highly exothermic reaction. Once all the AlCl3 was added, the resulting amber-looking liquid was left to gently stir overnight in the glove box. The liquid was then filtered to remove any un-dissolved AlCl3. The resulting acidic N-butyl-pyridinium chloroaluminate was used as the catalyst for the alkylation of isopentane with ethylene.
Isopentane and ethylene batch alkylation was typically run at 50° C. with paraffin/olefin molar ratio of about 4. Under nitrogen atmosphere in a glove box, an autoclave vessel was charged with ionic liquid catalyst and anhydrous isopentane. The autoclave was then sealed and transferred to a hood and affixed to an overhead stirrer. Then, ethylene gas was introduced to the vessel. The autogenic pressure of the vessel usually rises to 2000 kPa to 24000 kPa depending on the amount of ethylene gas introduced into the autoclave. Once the reaction begins stirring (˜1200 rpm), the pressure quickly drops down to ˜900 kPa to 1100 kPa. The reaction is allowed to continue and stir until the pressure drops to 0 kPa to 70 kPa. Then, the stirring is stopped and the heating mantle is quickly removed. The autoclave is then cooled down to room temperature using a cooling coil. Then, a gas sample was drawn and the reactor is vented and weathered to relieve the system from any remaining gas. The resulting solution is a biphasic with the product and excess isopentane phase is on top while the dense ionic liquid-catalyst phase is on the bottom. The top phase is then decanted and saved for analysis. The bottom phase is either recycled for further use or neutralized with water. Chemical analysis of the products in excess isopentane is usually done by gas chromatography analysis.
Ethylene (9.5 gm) was alkylated with isopentane (103 gm) in 20 gm butylpyridinium chloroaluminate ionic at 50° C. and the autogenic pressure in a closed 300 cc autoclave fitted with an overhead stirrer and a cooling coil. The reaction was allowed to stir at ˜1200 rpm until no significant drop in pressure was noticeable. Table 1 below shows the reaction results.
The reaction above was repeated in a fresh ionic liquid (19.6 gm) but this time 5 HCl (0.35 gm) was added as a co-catalyst (promoter) with 102.7 gm isopentane and 9.7 gm ethylene. The reaction was run at 50° C and autogenic pressure and 1200 rpm stirring. The reaction was terminated when no further pressure drop was noticeable. With HCl the reaction was noticeably exothermic. Table 1 below shows the results of the reaction.
The results from isopentane/ethylene alkylation are excellent and most of the products are in the desired alkylates range where C7s constitute the major fraction of the product mixture. Very little heavy products were produced.
Example 4 shows that addition of HCl as a co-catalyst enhances the activity of the ionic liquid catalyst and changes the product selectivity. When HCl was added as a co-catalyst, the reaction was done at much shorter time (completed in 5 minutes) and slight change in product selectivity was observed.
Other chloroaluminate ionic liquid catalysts with quaternary ammonium or 5 amine halide salt can perform the same alkylation chemistry. Table 2 below compares the alkylation results of isopentane with ethylene in different chloroaluminate ionic liquid catalysts. Quaternary ammonium or amine salts used are 1 -butyl-pyridinium (BPy), 4-methyl-1-butyl-pyridinium (MBPy), 1-butyl-4-methyl-imidaazolium (BMIM) and tributyl-methyl-ammonium (TBMA) chloroaluminates. The reactions were all conducted at 50° C. and autogenic pressure at a feed paraffin/olefin molar ratio of 4, in 20 gm ionic liquid for 1 hour.
The results above indicate that conversion of ethylene and product selectivity are affected by the catalyst selection. A chloroaluminate catalyst made with tributyl-methyl-ammonium is less active than the other three catalysts. Chloroaluminate catalysts made with hydrocarbyl substituted pyridinium chloride or a hydrocarbyl substituted imidazolium chloride shows high activity and good selectivity.
Evaluation of ethylene alkylation with isopentane was performed in a 100 cc continuously stirred tank reactor. 4:1 molar ratio of isopentane and ethylene mixture was fed to the reactor while vigorously stirring at 1600 rpm. An Ionic liquid catalyst was fed to the reactor via a second inlet port targeting to occupy 15 vol % in the reactor. A small amount of anhydrous HCl gas was added to the process (10:1 molar ratio of catalyst to HCl). The average residence time for the combined volume of feeds and catalyst was about 40 minutes. The outlet pressure was maintained at 2300 kPa using a backpressure regulator. The reactor temperature was maintained at 50° C. The reactor effluent was separated in a 3-phase separator into C4− gas; alkylate hydrocarbon phase, and the ionic liquid catalyst. Operating conditions and yield information are summarized in Table 3.
This alkylation process is highly selective in that 78.6% of the converted product is C7 isoparaffins. Detailed compositional analysis of the alkylate gasoline indicates the C7 fraction is nearly entirely derived from 2,3- and 2,4-dimethylpentane. 2,3-dimethylpentane and tri-methylbutanes are desirable isomers for high-octane gasoline (91 and 112 RON, respectively).
The hydrocarbon product was distilled to separate n-pentane and higher boiling alkylate gasoline (30° C.+) fraction and properties of the alkylate gasoline were measured or estimated. Research octane number was calculated based on GC composition and research octane number of pure compounds assuming volumetric linear blending. Blending octane numbers were measured at 7.5% and 15% blending level, then extrapolated to 100%. RVP and average density were estimated using the GC data assuming linear molar blending. T10, T50 and T90 were measured using ASTM D2887 simulated distillation.
The product property data shows that by employing the process of the present invention, high RVP isopentane (20 RVP) was converted to alkylate gasoline having a low RVP of 2.5. The high-octane (91 blending RON) and excellent boiling point distribution are other desirable features of the gasoline blending components prepared in accordance with the present method. To achieve the high-octane, it is preferable to maintain the 2,3-dimethylpentane selectivity above 40% relative to the total C7 yield.
Propylene alkylation with isopentane was performed via a similar procedure to that described in Example 6 except different process conditions were used. 8:1 molar ratio of isopentane and propylene mixture was fed to the reactor, at 10° C. reactor temperature and 7 vol % of catalyst. A summary of operating conditions and yield information are presented in Table 5.
The hydrocarbon product was distilled to generate n-pentane and higher boiling alkylate gasoline (30° C.+) fraction and properties of the alkylate gasoline were measured or estimated, and reported in Table 6.
The product property data shows that employing a process according to the present invention high RVP isopentane (20 RVP) was converted to alkylate gasoline having a low RVP of 1.0. The high-octane (82 RON), and excellent boiling point distribution are other desirable features of gasoline blending components prepared in accordance with the present invention.
Evaluation of C4 olefin alkylation with isobutane was performed in a 100 cc continuously stirred tank reactor. 8:1 molar ratio of isobutane and 2-butene mixture was fed to the reactor while vigorously stirring at 1600 RPM. An Ionic liquid catalyst was fed to the reactor via a second inlet port targeting to occupy 10-15 vol % in the reactor. A small amount of anhydrous HCl gas was added to the process. The average residence time (combined volume of feeds and catalyst) was about 8 minutes. The outlet pressure was maintained at 100 psig using a backpressure regulator. The reactor temperature was maintained at 0° C. using external cooling. The reactor effluent was separated in a 3-phase separator into C4− gas, alkylate hydrocarbon phase, and the ionic liquid catalyst. Detailed composition of alkylate gasoline was analyzed using gas chromatography. Research Octane number was calculated based on GC composition and Research Octane number of pure compounds assuming volumetric linear blending. The operating conditions and performance are summarized in Table 7.
The results in Table 7 show that high octane alkylate can be obtained with n-butylpyridinium chloroaluminate ionic liquid catalyst. With 2-butene, over 95% of the C8 fraction is composed of trimethylpentanes having a RON of about 100.
The process of the present invention can decrease the amount of excess fuel gas production in a refinery by converting ethylene in FCC offgas. This aspect of this invention is shown in this Example using a typical FCC offgas data from a refinery, as summarized in Table 8.
This refinery generates 26 million standard cubic feet (MMSCFD) of fuel gas from an FCC unit daily and the stream contains 15 vol % ethylene. Using a process according to the present invention results in converting the ethylene stream into high-octane gasoline blending component by alkylating the stream with isopentane or isobutane. The amount of fuel gas from the ethylene extraction unit is reduced to 21 MMSCFM, thus lowering the burden of fuel gas processing equipment. In this case, approximately a 19 percent reduction of fuel gas is feasible.
Extracting ethylene or the C2+ stream will improve the purity of hydrogen in FCC offgas as shown in Table 8, from 36% to 44%. Further upgrading of the reject stream can be achieved by recovering pure hydrogen gas with use of a hydrogen recovery unit such as a pressure-swing adsorption (PSA) unit or a membrane unit. By combining extraction of ethylene and hydrogen recovery, the amount of fuel gas is reduced substantially. In this case, up to a 55% reduction of fuel gas can be realized relative to the base case. In addition, 9 MMSCFD of hydrogen gas can be recovered. Considering the very stringent environmental regulations that are associated with fuel gas production and storage of hydrogen in refineries, the benefits of fuel gas reduction and hydrogen production that are made possible by using the present invention are significant and highly desirable.
There are numerous variations on the present invention which are possible in light of the teachings and supporting examples described herein. It is therefore understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described or exemplified herein.
